1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacture thereof and, more particularly, to a technique to provide a semiconductor device which is readily manufactured without epitaxial growth process and is highly reliable.
2. Description of the Prior Art
A conventional transistor which is one of the semiconductor device is shown in FIG. 1. In this transistor, an N.sup.- type epitaxially grown layer 31 is formed on an N.sup.+ type silicon substrate 30. Further, the N.sup.- type epitaxially grown layer 31 is provided with a P type diffusion layer 32 and an N type diffusion layer 33, to which a base electrode 34 and an emitter electrode 35 are attached, respectively. In addition, a base terminal T1 and an emitter terminal T2 are connected to the base electrode 34 and the emitter electrode 35, respectively. Also, a collector electrode 36 is formed on the bottom surface of the N.sup.+ type silicon substrate 30 whereto a collector terminal T3 is connected.
Such a transistor is manufactured as follows. First, the N.sup.- type epitaxially grown layer 31 is grown on the N.sup.+ type silicon substrate 30 by epitaxial growth. The epitaxial growth is performed in such a way that the silicon substrate, along with a proper amount of gas, is heated to a high temperature of 1000.degree. C. or more. This treatment allows the N.sup.- type epitaxially grown layer 31 to structurally adjoin the N.sup.+ type silicon substrate 30.
Next, the P type diffusion layer 32 and the N type diffusion layer 33 are formed in the N.sup.- type epitaxially grown layer 31. Here, the N.sup.- type epitaxially grown layer 31 is provided for forming the P type diffusion layer 32 and the N type diffusion layer 33. This is because the N.sup.+ type layer contains a higher density of impurities and has a lower resistance value as compared with the N.sup.- type layer. This means that desired electrical characteristics of a transistor may not be achieved in the case where a device is formed in the N.sup.+ type layer due to failure in having high breakdown voltage. Also, the P type diffusion layer 32 and the N type diffusion layer 33 are formed by implanting ions or other impurities and performing thermal diffusion.
Subsequent to device formation in the P type diffusion layer 32 or other layers, an insulating layer 37 is provided on the top surface of the N.sup.- type epitaxially grown layer 31. Thereafter, the base electrode 34 and the emitter electrode 35 are formed, to which the base terminal T1 and the emitter terminal T2 are connected, respectively. Further, the collector electrode 36 and the collector terminal T3 are formed on the bottom surface of the N.sup.+ type silicon substrate 30.
In a semiconductor device such as a transistor, the N.sup.+ type silicon substrate 30 is formed rather thick in order to give a sufficient strength to the device, as shown in FIG. 1. Moreover, the N.sup.+ type silicon substrate 30, which is an N.sup.+ type layer, contains a higher density of impurities and has a lower resistance value, as described earlier. Consequently, the semiconductor device holds a sufficient strength due to the thick substrate, maintaining its collector resistance at a low level.
The conventional semiconductor device described above, however, involves the following problem. In the transistor shown in FIG. 1, epitaxial growth is performed to grow the N.sup.- type epitaxially grown layer 31 on the N.sup.+ type silicon substrate 30. The epitaxial growth at this step is such that it requires a long time for crystal growth, decreasing the productivity in the manufacture of transistors.
In order to solve this problem, a way of forming a device directly in the surface of an N.sup.- type silicon substrate, which is adopted instead of the N.sup.+ type silicon substrate, has been thought. This method permits the process of epitaxial growth to be eliminated, because the whole silicon substrate is originally arranged as N.sup.- type.
Nevertheless, the method mentioned above involves further problem. The resistance value of the N.sup.- type layer is higher than that of the N.sup.+ type layer, as described earlier. Hence, in the case where the N.sup.+ type silicon substrate 30 shown in FIG. 1 is replaced with the N.sup.- type silicon substrate, a collector formed on the bottom surface of the substrate has such high resistance that a saturation current characteristic and a frequency characteristic of a transistor may be deteriorated. Although the collector resistance may be diminished by reducing transistor thickness and thereby placing the collector electrode 36 near the P type diffusion layer 32, this measure would lead to decreasing the strength of a transistor involved.